Thermoplastic Starch Compositions

ABSTRACT

The present invention relates to improvements in prevention of discoloration of thermoplastic starch materials and their blends with other thermoplastic materials.

FIELD OF THE INVENTION

The present invention relates to improvements in prevention ofdiscoloration of thermoplastic starch materials and their blends withother thermoplastic materials.

BACKGROUND TO THE INVENTION

Thermoplastic starch compositions are often used to make final plasticproducts. The thermoplastic starch composition is heated and formed intothe products using known techniques such as injection moulding,extrusion blow moulding, thermoforming, injection stretch blow moulding,cast film extrusion, blown film extrusion and fiber spinning/extrusion.Often the thermoplastic starch compositions have many of thecharacteristics of other plastic compositions, such as polyethylene orpolypropylene, however, they have the added advantage that they can besourced from renewable resources and, are often readily biodegradable.This provides for more environmentally friendly plastic products.

However, a problem with thermoplastic starch compositions, is thetendency for discoloration upon heating. There are various steps andprocesses in the formation and processing of the thermoplastic starchcomposition that require it to be heated. Examples include,destructuring of the starch, and heating of the thermoplastic starchcomposition during product forming processes such as injection molding,extrusion blow moulding, thermoforming, injection stretch blow moulding,cast film extrusion, blown film extrusion and fiber spinning/extrusion.Discoloration can be further increased when thermoplastic starch isblended with other thermoplastic materials or fillers, especially whenhigh thermal and mechanical energy processes such as compounding areused. Discoloration can be seen as a yellow or brown tint, which isviewed as an undesirable characteristic of the end use product by theconsumer. In its most extreme form, discoloration can result in blackmaterial. Discoloration can also occur in the final product if it issubjected to direct heat or UV, such as being left in direct sunlight.

The discoloration of thermoplastic starch can be attributed to numerouscomplex and interconnected non-enzymatic ‘browning reactions’.‘Maillard-type’ reactions are one type of non-enzymatic browningreaction. These reactions occur between residual amino compounds andreducing groups present in the starch compounds. This reaction occurs atintermediate temperatures and in the presence of moisture. Maillardreactions are known to be inhibited by compounds such as sulfite (sodiumsulfite, sodium bisulfite, and sodium metabisulfite). Oxidation isanother type of browning reaction that occurs at elevated temperaturesin oxygen rich environments. Caramelization reactions are yet anothernon-enzymatic browning reaction that occurs in starch and otherpolysaccharides. Caramelization reactions are complex reactions thatoccur under elevated temperatures and low moisture content.Caramelization reactions are catalyzed by numerous agents and the typeof caramel produced is characterized by the catalyst used. For example,type I caramels are produced under basic conditions and type II caramelsare produced under basic conditions with a sulfite compound such assodium sulfite.

Therefore, a compound such as a sulfite could lower discolorationassociated with the Maillard reactions, but increase discolorationassociated with Caramelization reactions, especially under conditions ofhigh temperature and low moisture.

Caramelization reactions can be inhibited by low pH conditions. However,low pH conditions catalyze starch hydrolysis, which leads to a reductionin starch molecular weight. A reduction in starch molecular weight canresult in a decrease in viscosity of the molten thermoplastic starch.This may negatively affect certain polymer processing operations such asinjection stretch blow molding, extrusion blow moulding, cast filmextrusion, and blown film extrusion as the low viscosity thermoplasticstarch composition cannot follow the contours of the mold and so resultsin excessively thick and/or thin areas of the final product. It may alsonegatively affect the mechanical properties of the final product, suchas tensile strength.

WO2008014573 discloses the use of the reducing agents sodium sulfite,sodium bisulfite and metabisulfite as anti-discoloration agents forthermoplastic starch. However, as mentioned above, sodium sulfite andsodium bisulfite are known to catalyze the formation of type II caramelsespecially under basic conditions, elevated temperatures, and lowermoisture content. It has been found that under the elevatedtemperatures, high energy, and low moisture processing associated withthermoplastic starch production, these agents are not sufficient toreduce discoloration and actually contribute themselves to additionalbrowning under certain conditions.

There is a need in the art for thermoplastic starch compositionsexhibiting less discoloration during processing and in the final plasticproduct, than is seen using known thermoplastic starch compositions.

There is a further need for thermoplastic starch compositions thatexhibit less discoloration yet also maintain a desirable molecularweight starch during processing.

It was surprisingly found that the thermoplastic starch compositions ofthe present invention resulted in the production of thermoplastic starchmaterials exhibiting less discoloration than is seen using knownthermoplastic starch compositions. It was also surprisingly found thatthe thermoplastic starch compositions of the present invention,following extensive thermal/mechanical processing, comprised starch of ahigher molecular weight, than is seen in other thermoplastic starchcompositions subjected to the same processing.

SUMMARY OF THE INVENTION

The present invention is to a thermoplastic starch composition, a methodof making a thermoplastic starch composition and a use of a reducingagent in a thermoplastic starch composition.

An aspect of the present invention is a thermoplastic starch compositioncomprising,

-   -   from 40% to 96% by weight of the thermoplastic starch        composition, of a starch;    -   from 1% to 40% by weight of the thermoplastic starch        composition, of a plasticizer;        and wherein;        the thermoplastic starch composition comprises 0.01% to 5% by        weight of the thermoplastic starch composition, of a reducing        agent, wherein the reducing agent has a redox potential, wherein        the redox potential has a value of from −50 mV to −1200 mV,        preferably from −100 mV to −1200 mV, more preferably from −150        mV to −1200 mV, the redox potential being measured in an aqueous        solution comprising 1% reducing agent, 1% citric acid, and 1%        tribasic potassium citrate by weight of the aqueous solution,        and wherein the aqueous solution is at a temperature of 80° C.

Another aspect is a method of preparing the thermoplastic starchcomposition comprising the steps;

-   -   adding from 1% to 40% by weight of the thermoplastic starch        composition, of a plasticizer;    -   adding 0.01% to 5% by weight of the thermoplastic starch        composition, of a reducing agent, wherein the reducing agent has        a redox potential, wherein the redox potential has a value of        from −50 mV to −1200 mV, preferably from −100 mV to −1200 mV,        more preferably from −150 mV to −1200 mV, the redox potential        being measured in an aqueous solution comprising 1% reducing        agent, 1% citric acid and 1% tribasic potassium citrate by        weight of the aqueous solution, and wherein the aqueous solution        is at a temperature of 80° C.    -   adding, from 40% to 96% by weight of the thermoplastic starch        composition, of a starch;    -   mixing the thermoplastic starch composition;    -   passing the thermoplastic starch composition through a        compounder;    -   removing excess water present.

Yet another aspect of the present invention is the use of a reducingagent in a thermoplastic starch composition to reduce discoloration of athermoplastic starch composition and material processed from thethermoplastic starch composition, wherein the reducing agent has a redoxpotential, wherein the redox potential has a value of from −50 mV to−1200 mV, preferably from −100 mV to −1200 mV, more preferably from −150mV to −1200 mV, the redox potential being measured in an aqueoussolution comprises, 1% reducing agent, 1% citric acid and 1% tribasicpotassium citrate by weight of the aqueous solution, and wherein theaqueous solution is at a temperature of 80° C.

DETAILED DESCRIPTION OF THE INVENTION Starch

The thermoplastic starch composition of the present invention comprisesfrom 40% to 96% by weight of the thermoplastic composition, of a starch.In one embodiment, the thermoplastic starch composition comprises from50% to 80%, preferably from 60% to 70% by weight of the thermoplasticcomposition, of a starch. Starch is a low cost naturally occurringbiopolymer. Preferably, the starch is selected from the group comprisingnatural starch, modified starch, or mixtures thereof.

In one embodiment, the starch used in the present invention is in anative or natural state. By ‘native or natural starch’, we herein meanthe molecular structure of the starch has not deliberately been subjectto any modification by chemical or any other means. Preferably,naturally occurring starches comprise starch selected from the groupcomprising, corn starch (e.g. waxy maize starch), potato starch, sweetpotato starch, wheat starch, sago palm starch, tapioca starch, ricestarch, soybean starch, arrow root starch, bracken starch, lotus starch,cassava starch, high amylose corn starch, and commercial amylose powder,or mixtures thereof. It is advantageous that the natural starch isderived from agricultural sources, which offer the advantages of beingabundant in supply, easy to replenish and inexpensive in price.Preferably, the natural starch comprises starch selected from the groupcomprising corn starch (including waxy maize starch), wheat starch,potato starch, and mixtures thereof. In one embodiment, the naturalstarch comprises corn starch. In another embodiment, the natural starchcomprises waxy maize starch. Natural corn starch is favourable as it ischeaper than other natural starches such as tapioca. Furthermore,different sources of corn also present different amylose to amylopectinratios, which can impact the mechanical properties, such as tensilestrength, of the final thermoplastic starch product. In anotherembodiment, the corn starch can be genetically modified corn starch.

It may be preferable to use natural starch rather than modifiedstarches, as natural starches are more economical. Natural, unmodifiedstarch generally has a very high average molecular weight and a broadmolecular weight distribution (e.g. natural corn starch has an averagemolecular weight of up to about 60,000,000 grams/mole).

In another embodiment, the starch used in the present invention is amodified starch. Modified starch is defined as non-substituted, orsubstituted, starch that has had its native molecular weightcharacteristics changed (i.e. the molecular weight is changed but noother changes are necessarily made to the starch). Molecular weight canbe modified, preferably reduced, by any of numerous techniques which arewell known in the art. These include, for example, chemicalmodifications of starch including, but not limited to acid or alkalihydrolysis, acid reduction, oxidative reduction, enzymatic reduction,physical/mechanical degradation (e.g. via thermomechanical energy inputof the processing equipment), or combinations thereof. Thethermomechanical method and the oxidation method offer an additionaladvantage when carried out in situ. The exact chemical nature of thestarch and molecular weight reduction method is not critical as long asthe average molecular weight is provided at the desired level or range.Such techniques can also reduce molecular weight distribution. Specificmolecular ranges can impart beneficial mechanical properties to thefinal thermoplastic starch product.

Optionally, substituted starch can be used. Chemical modifications ofstarch to provide substituted starch include, but are not limited to,etherification and esterification. For example, methyl, ethyl, or propyl(or larger aliphatic groups) can be substituted onto the starch usingconventional etherification and esterification techniques known in theart. Such substitution can be done when the starch is in naturalgranular form or after it has been destructured. The term “destructuredstarch” herein means starch that has lost most of its crystalline orderand so no longer has its natural structure. This results in the starchhaving a lower average molecular weight. The degree of substitution ofthe chemically substituted starch is typically, but not necessarily,from about 0.01 to about 3.0, and can also be from about 0.01 to about0.06.

Starch pre-gels can also be used. Pre-gelled starches are starches thathave been partially or totally destructured in a separate processtypically using water. In another embodiment, the starch is destructuredduring processing to produce the thermoplastic starch composition.Preferably, at least about 50% of the starch is destructured starch,more preferably at least about 80% of the starch is destructured starch,most preferably at least about 90% of the starch is destructured starch.In a most preferred embodiment, about 100% of the starch is destructuredstarch.

Starch can be destructured in a variety of different ways. The starchcan be destructurized with a solvent. For example, starch can bedestructurized by subjecting a mixture of the starch and solvent toheat, which can be under pressurized conditions and shear, to gelatinizethe natural starch, leading to destructurization. Typically, water isalso used as a solvent in the destructuring process.

Plasticizer

The thermoplastic starch composition comprises a plasticizer. For starchto flow and to have the characteristics when molten to allow it to beprocessed like a conventional thermoplastic polymer, a plasticizer needsto be present. In general, the plasticizers should be substantiallycompatible with the polymeric components of the present invention withwhich they are intermixed. As used herein, the term “substantiallycompatible” means when heated to a temperature above the softeningand/or the melting temperature of the composition, the plasticizer iscapable of forming a homogeneous mixture with the components present inthe composition in which it has been mixed.

The plasticizer present during further melt processing may be the sameas the solvent used to destructure the starch, as solvents can also actas plasticizers. Hence, the solvent may remain in the destructuredstarch component to subsequently function as a plasticizer for thestarch, or may be removed and substituted with a different plasticizerin the thermoplastic starch composition. The plasticizers may alsoimprove the process flexibility of the final products, which is believedto be due to the fact that they lower the glass transition temperatureof the composition (hence less energy needs to be used during heating).

Varieties of plasticizing agents that can also act as solvents todestructure starch are described herein. These include the low molecularweight or monomeric plasticizers, such as but not limited tohydroxyl-containing plasticizers, including but not limited to thepolyols, e.g. polyols such as mannitol, sorbitol, and glycerol. Wateralso can act as a solvent for starch, and can be used to destructure thestarch too.

Plasticizers that are added to the thermoplastic starch composition,(rather than as solvents to destructure the starch and are left in thecomposition) can include monomeric compounds and polymers. The polymericplasticizers will typically have a molecular weight of about 100,000g/mol or less. Polymeric plasticizers can include block copolymers andrandom copolymers, including terpolymers thereof. In certainembodiments, the plasticizer is a low molecular weight plasticizer. Inone embodiment, the plasticizer has a molecular weight of about 20,000g/mol or less. In another embodiment, the plasticizer has a molecularweight of about 5,000 g/mol or less. In yet another embodiment, theplasticizer has a molecular weight of about 1,000 g/mol or less.

In one embodiment, the plasticizer is selected from the group comprisingmonomeric compounds and polymers, organic compounds having at least onehydroxyl group, hydroxyl polymeric plasticizers, hydrogen bondingorganic compounds, aliphatic acids and mixtures thereof.

The plasticizer can be, for example, an organic compound having at leastone hydroxyl group, including polyols having two or more hydroxyls.Preferably, hydroxyl plasticizers are selected from the group comprisingsugars such as glucose, sucrose, fructose, raffinose, maltodextrose,galactose, xylose, maltose, lactose, mannose erythrose, andpentaerythritol; sugar alcohols such as erythritol, xylitol, malitol,mannitol and sorbitol; polyols such as glycerol (glycerin), ethyleneglycol, propylene glycol, dipropylene glycol, butylene glycol, hexanetriol, and the like, and polymers thereof; and mixtures thereof.Preferably, the hydroxyl plasticizers are selected from the groupcomprising glycerol, mannitol, sorbitol, and mixtures thereof.

Also useful herein are hydroxyl polymeric plasticizers. Preferably,hydroxyl polymeric plasticizers are selected from the group comprisingpoloxamers (polyoxyethylene/polyoxypropylene block copolymers),poloxamines (polyoxyethylene/polyoxypropylene block copolymers ofethylene diamine) and mixtures thereof. These copolymers are availableas Pluronic® from BASF Corp., Parsippany, N.J. Suitable poloxamers andpoloxamines are available as Synperonic® from ICI Chemicals, Wilmington,Del., or as Tetronic® from BASF Corp., Parsippany, N.J.

Also suitable for use herein as plasticizers are hydrogen bondingorganic compounds, including those which do not have an hydroxyl group.Preferably, hydrogen bonding organic compounds are selected from thegroup comprising urea and urea derivatives; anhydrides of sugar alcoholssuch as sorbitol; animal proteins such as gelatin; vegetable proteinssuch as sunflower protein, soybean proteins, cotton seed proteins; andmixtures thereof. In another embodiment, plasticizers are selected fromthe group comprising phthalate esters, dimethyl and diethylsuccinate andrelated esters, glycerol triacetate, glycerol mono and diacetates,glycerol mono, di, and tripropionates, butanoates, stearates, lacticacid esters, citric acid esters, adipic acid esters, stearic acidesters, oleic acid esters, and other fatty acid esters which arebiodegradable, and mixtures thereof. In yet another embodiment, theplasticizer is an aliphatic acid selected from the group comprisingethylene acrylic acid, ethylene maleic acid, butadiene acrylic acid,butadiene maleic acid, propylene acrylic acid, propylene maleic acid,and other hydrocarbon based acids and mixtures thereof.

Preferably, the plasticizer is selected from the group comprisingglycerol, sorbitol, or mixtures thereof. Most preferably, theplasticizer is glycerol.

Other non-limiting examples of plasticizers include Ecoflex, availablefrom BASF Corp or other equivalent plasticizing polyesters.

The plasticizer of the present invention is present from 1% to 40% byweight of the thermoplastic starch composition. This concentrationincludes all plasticizer present in the composition, including anyplasticiser that has been carried over following destructuring of thestarch, where it acted as a solvent.

Reducing Agent

The thermoplastic starch composition of the present invention comprises0.01% to 5% by weight of the thermoplastic starch composition, of areducing agent. By reducing agent we herein mean a compound capable ofreducing another chemical species, itself being oxidized in thereaction. A reducing agent is characterized by its redox potential,herein reported in mV. The full method for determining the redoxpotential of the reducing agent is detailed under the heading Method A.

The thermoplastic starch compositions of the present invention comprisea reducing agent having a redox potential, wherein the redox potentialhas a value from −50 mV to −1200 mV, preferably from −100 mV to −1200mV, more preferably from −150 mV to −1200 mV, the redox potential beingmeasured in an aqueous solution comprising 1% reducing agent, 1% citricacid, and 1% tribasic potassium citrate; by weight of the aqueoussolution, and wherein the aqueous solution is at a temperature of 80° C.

Preferably, the reducing agent is a sulfur-comprising reducing agent.More preferably, the sulfur-comprising reducing agent is selected fromthe group comprising sulfinic acid derivatives or salts thereof,sulfonic acid derivatives or salts thereof, or mixtures thereof.

Sulfinic acid derivatives or salts thereof have the general formulaMSO₂, wherein R is selected from the group comprising aliphatic groups,aromatic groups, hydroxyl groups, amino groups, imino groups,amino-imino groups, thiol groups, —MSO₂, and M is selected from thegroup consisting of H, NH₄ ⁺, or monovalent metal ion. In a preferredembodiment, the sulfinic acid derivative has the general formula;

wherein, M is selected from the group consisting of H, NH₄ ⁺, monovalentmetal ion or a divalent metal ion; R¹ is selected from the groupconsisting of OH or NR⁴R⁵, wherein R⁴ and R⁵ are independently H orC₁-C₆ alkyl groups; R² is selected from the group consisting of H, alkylgroup, alkenyl group, cycloalkyl group or aryl group, each group having1, 2 or 3 substituents selected from the group consisting ofC₁-C₆-alkyl, OH, O—C₁-C₆-alkyl, halogen and CF₃; R³ is selected from thegroup consisting of COOM, SO₃M, COR⁴, CONR⁴R⁵ or COOR⁴, wherein M, R⁴and R⁵ are defined as above, and wherein where R² is an aryl group, R³is H. These structures can be produced following the teachings of U.S.Pat. No. 6,211,400 involving adducts of sodium dithionite and aldehydes.In one embodiment, the sulfinic acid derivatives or salts thereof areselected from the group comprising sodium hydroxymethylsulfonate,disodium 2-hydroxy-2-sulfinatoacetic acid, sodium dithionite, thioureadioxide and mixtures thereof.

Sulfonic acid derivatives or salts thereof have the general formula,MSO₃R where R is selected from the group comprising aliphatic groups,aromatic groups, hydroxyl groups, amino groups, imino groups,amino-imino groups, thiol groups, —MSO₂, and M is selected from thegroup consisting of H, NH₄ ⁺, or monovalent metal ion. In oneembodiment, the sulfonic acid derivative is disodium2-hydroxy-2-sulfonatoacetic acid.

In one embodiment, the sulfur-comprising reducing agent is selected fromthe group comprising sodium hydroxymethylsulfonate, disodium2-hydroxy-2-sulfinatoacetic acid, sodium dithionite, thiourea dioxide,disodium 2-hydroxy-2-sulfonatoacetic acid, and mixtures thereof.

In another embodiment, the sulfur-comprising reducing agent is selectedfrom the group comprising disodium 2-hydroxy-2-sulfinatoacetic acid,disodium 2-hydroxy-2-sulfonatoacetic acid, and mixtures thereof.

The sulfur-comprising reducing agent of the present invention is presentbetween about 0.01% and 5%, preferably between 0.05% and 2%, morepreferably between 0.05% and 1%, even more preferably between 0.1% and0.5% by weight of the thermoplastic starch composition.

While not wishing to be bound by theory, it is believed that thereducing agents of the present invention minimize Maillard and oxidationtype reactions, and so reduce the amount of discoloration in both thethermoplastic starch composition and articles made from thethermoplastic starch composition. However, unlike the reducing agentsdescribed in the prior art, the reducing agents of the present inventionalso minimize discolouration due to caramelization reactions.Furthermore, it is believed that the reducing agents of the presentinvention reduce or minimize starch hydrolysis at low pH, thus enablinglow pH to also be used to minimize caramelization based discolouration.

Acidic Agent

In one embodiment, the thermoplastic starch compositions comprise anacidic agent or acid/base conjugate pair. Preferably, the acidic agentis selected from the group comprising citric acid, tribasic potassiumcitrate or mixtures thereof.

Caramelization reactions can be inhibited by lowering pH during theheating stage of starch processing. However, lower pH has thepotentially negative effect of catalyzing starch hydrolysis. It wassurprisingly found that the presence of the reducing agents of thepresent invention also reduced the hydrolysis associated with processingof thermoplastic starch compositions especially when low pH/acidicprocessing conditions were used. Therefore, low pH combined with thereducing agents of the present invention can also be used to moreeffectively minimize caramelization browning reactions whilst minimizingthe negative impact on the molecular weight of the starch.

Other Ingredients

Optionally, other ingredients may be incorporated into the thermoplasticstarch composition. These optional ingredients may be present inquantities of from 49% or less, or from 0.1% to 30%, or from 0.1% to 10%by weight of the composition. The optional materials may be used toassist in processing of the thermoplastic starch composition and/or tomodify physical properties such as elasticity, tensile strength,modulus, or other properties or benefits of the final product. Otherproperties or benefits include, but are not limited to, stabilityincluding oxidative stability, brightness, color, flexibility,resiliency, processing aids, viscosity modifiers, and odor control. Apreferred processing aid is magnesium stearate. A preferred odor controlagent is zinc carbonate. Another optional material that may be desired,particularly in the starch component, is ethylene acrylic acid,commercially available as Primacor by Dow.

In one embodiment, the thermoplastic starch comprises anotherthermoplastic material. In a preferred embodiment, the thermoplasticstarch composition comprises a polyolefin material, selected from thegroup comprising polyethylene, polypropylene, polyethylene terephthalateand mixtures thereof. In another embodiment, the thermoplastic starchmaterial comprises a polyolefin material, wherein the polyolefinmaterial is sourced from renewable resources such as cellulose, starchand/or sugar containing crops.

Preparation of the Thermoplastic Starch Composition

Thermoplastic starch compositions of the present invention can beproduced in numerous ways to provide a destructured starch combined witha plasticizer. Water in some form is typically used as the destructuringagent. Preferably, the thermoplastic starch compositions of the presentinvention are produced using either the dry process or the slurryprocess.

In the dry process method, water naturally present in the starch is usedas the primary destructuring agent. In this process, additional water isnot normally added and the starch is fed as a powder and/or a mixturewith the plasticizer. Typically, chemically modified starches are usedin this type of process to enable easier destructuring due to the lackof extra water, which would be necessary to facilitate destructuring ofnatural or unmodified starch.

In the slurry process method, excess water is added to the starch tofacilitate destructuring. Typically, the starch, water, and optionallythe plasticizer are premixed into the form of a slurry. Typically,natural starch or “lightly modified starch” are used in this process dueto their lower cost and due to the fact that excess water is added,destructuring is easier than in the dry process. Lightly modifiedstarches are starches that have not been extensively modified,therefore, they still need to be destructured. In this process, theextra water must be removed, which requires more heat energy to beexpended to produce the desired product. Therefore, thermoplastic starchcompositions prepared using slurry process are more subject todiscoloration issues than thermoplastic starch compositions made usingthe dry process.

The method of preparing the thermoplastic starch compositions of thepresent invention comprises the steps of;

-   -   adding from 1% to 40% by weight of the thermoplastic starch        composition, of a plasticizer;    -   adding 0.01% to 5% by weight of the thermoplastic starch        composition, of a reducing agent, wherein the reducing agent has        a redox potential, wherein the redox potential of the reducing        agent is from −50 mV to −1200 mV, preferably from −100 mV to        −1200 mV, more preferably from −150 mV to −1200 mV, the redox        potential being measured in an aqueous solution comprising, 1%        reducing agent, 1% citric acid, and 1% tribasic potassium        citrate by weight of the aqueous solution, and wherein the        aqueous solution is at a temperature of 80° C.    -   adding, from 40% to 96% by weight of the thermoplastic starch        composition, of a starch;    -   mixing the thermoplastic starch composition;    -   passing the thermoplastic starch composition through a        compounder;    -   removing excess water present.

During the process, the thermoplastic starch composition is passedthrough a compounder, where it is subjected to thermal and mechanicalenergy typically under elevated pressure to keep the bound water in apredominately bound state. Following this, bound water is largelyremoved through application of thermal and mechanical energy understandard atmospheric conditions and/or vacuum. The plasticizer acts toplasticize the thermoplastic starch composition, and the starch isdestructured. The final material is a thermoplastic starch compositionthat can be further combined with other natural or synthetic polymers orcan be used in the current form to produce extruded and/or shapedproducts.

The slurry process differs from the dry process in that water is addedto aid with the hydrolysis of the starch, and to make the thermoplasticstarch composition easier to handle during processing. Preferably, thewater is added to the thermoplastic starch composition before thethermoplastic starch composition is mixed. Preferably, sufficient wateris added to provide the thermoplastic starch composition with aviscosity that allows it to flow or be pumped into the compounder,making it easier to handle the thermoplastic starch composition duringprocessing. The amount of water will differ depending on the type andamount of the other ingredients added.

Since it is a slurry, the pH can be measured (when the flow propertiesof the slurry range from ‘water-like’ to more viscous ‘paste-like’consistency), and more accurately adjusted at this point than in the dryprocess, where it is impossible to measure the pH of the dryingredients. In one embodiment, the pH of the thermoplastic starchcomposition is adjusted to below 6, preferably below 4. In oneembodiment, the acidic agent or acid/base conjugate pair is added afterthe plasticizer is added, but before the reducing agent is added.Preferably, the acidic agent or acid/base conjugate pair is added insufficient quantity to adjust the pH of the thermoplastic starchcomposition to less than 6, preferably less than 4. In one embodiment,the pH of the thermoplastic starch is adjusted to between 6 and 1,preferably between 4 and 2.

In the case of the slurry process, most unbound and bound water areremoved following passing of the thermoplastic starch compositionthrough the compounder through application of thermal and mechanicalenergy under standard atmospheric conditions and/or vacuum.

Processing of the Thermoplastic Starch Composition

Thermoplastic starch compositions may be processed into complex forms byvarious processes, notably moulding processes, extrusion processes, etc.In these processes the thermoplastic starch composition is typicallyheated to a temperature above its melt temperature T_(m), so that thethermoplastic starch composition can be formed into the desired shape.Preferred processing temperatures are between about 60° C. and about300° C. In moulding processes, a mould which generally comprises two ormore parts is provided, which can be closed to form a mould cavity. Ininjection moulding processes the thermoplastic starch composition isinjected into the mould cavity. In blow moulding processes a heatedpreform or parison is placed within the mould and air is injected intothe preform or parison so that it expands within the mould cavity toform a hollow body. In extrusion processes the heated thermoplasticstarch composition is forced under pressure through an extrusion die.Many variations on these basic processes are practiced in industry, suchas, for example, injection stretch blow moulding, extrusion blowmoulding, film casting, film blowing, fiber spinning, etc.

Use of the Reducing Agent

One aspect of the present invention is the use of a reducing agent in athermoplastic starch composition to reduce discoloration of athermoplastic starch composition and material processed from thethermoplastic starch composition, wherein the reducing agent has a redoxpotential, wherein the redox potential has a value from −50 mV to −1200mV, preferably from −100 mV to −1200 mV, more preferably from −150 mV to−1200 mV, the redox potential being measured in an aqueous solutioncomprising, 1% reducing agent, 1% citric acid and 1% tribasic potassiumcitrate by weight of the aqueous solution, and wherein the aqueoussolution is at a temperature of 80° C.

Method A: Measurements of Redox Potential and pH

Approximately 20 g of citric acid and 20 g of tribasic potassium citratewere added to 1960 g of deionised water (resistance was greater than 17mΩ). The mixture was stirred for approximately 15 minutes and heated to80° C. The pH and redox potential were measured. Redox potential wasmeasured using an EcoSense ORP 15 probe manufactured by YSI. The redoxprobe had been calibrated using a saturated solution of Quinhydrone atpH 4. The pH was measured using an EcoSense pH 10 probe manufactured byYSI. The pH probe was calibrated with PerpHect buffer solutions fromOrion. Specifically, Buffer 4, 7, and 10 traceable to NIST standardswere used. Approximately 100 g of the buffered solution was added to a400 ml beaker. A small stir bar was added and the flask placed on astandard lab hot plate stirrer that actively controlled the temperatureto 80° C. Temperature and redox potential were measured until bothstabilized. 1 g of the reducing agent was added to the flask whilststirring. The mixture was allowed to stir for 30 seconds at 80° C. whilemeasuring redox and pH. After the initial 30 seconds, the redoxpotential and pH were recorded. The redox and pH electrodes were thenrinsed with fresh deionised water.

Method B: Production of Flat Discs for Optical Measurements

Thermoplastic starch or Thermoplastic starch and other thermoplasticsamples were pressed into thin films using a JRD Compression Molder withheated platens. The heated platens of the compression molder wereequilibrated to 170° C. Approximately 2.5 g of a thermoplastic starchcomposition was placed on a piece of Teflon coated woven fiberglasssheet (the dimensions of the fiberglass sheet were approximately sixinches square). The thermoplastic starch composition was heaped into asingle pile with a base diameter of about 1.5 inches. A stainless steelshim was used to assure a relatively uniform thickness. The outsidedimension of the shim was approximately six inches square. A rectangulararea was present inside the shim with dimensions of approximately 10 cmby 13 cm. The shim was placed around the Thermoplastic starchcomposition pile so that the pile was in the approximate center of theopen rectangular area of the shim. Another piece of Teflon coated wovenfiberglass was place on top of the pile. The fiberglass sheeting allowsfor the easy removal of the compressed thermoplastic starch composition.The shimmed thermoplastic starch composition sample was then placedinside the compression and the heated platens were slowly broughttogether using the hydraulic control of the molder. The thermoplasticstarch composition sample was then allowed to equilibrate at 170° C. for90 seconds. After 90 seconds the sample was compression molded byrapidly increasing the pressure of the shimmed thermoplastic starchcomposition sample to approximately 40 Bar. After 30 seconds thepressure was again rapidly increased back to 40 Bar for an additional 30seconds. After 60 seconds of compression, the pressure was released andthe samples were removed and allowed to cool. The compressedthermoplastic starch sample was then removed from the fiberglasssheeting and was placed between two sheets of paper and was stored in areclosable plastic bag to minimize moisture absorption. The compressedthermoplastic starch sample was allowed to equilibrate to roomtemperature for at least sixteen hours before color measurements weretaken.

Method C: Color Measurements of Flat Discs

Color measurements were obtained with a Minolta Spectrophotometer, ModelCM580d. The ‘white’ portion of a Leneta card was used as a commonbackground and as the reference point for ΔE calculations. ΔE is thecolor difference between a sample color and a reference color. Colormeasurements were taken using a D65 illuminant and a 10° observer. Aminimum of three measurements were taken for each of the compressedthermoplastic starch composition samples. L, a, b values (Hunter 1948 L,a, b color space) were averaged and reported along with ΔE values. ΔEvalues for the pure white Leneta card are zero and positive deviationsfrom zero indicate increased discoloration. Those skilled in the artwill know how to calculate the ΔE value. The efficacy of a givenanti-discoloration agent is related to the change in ΔE relative to acontrol sample produced using identical materials and process butwithout reducing agent and/or pH buffer. The color improvement index forthe sample, ζ, is calculated from the following equation:

$ϛ = {\frac{\left( {{\Delta \; E_{control}} - {\Delta \; E_{agent}}} \right)}{\Delta \; E_{control}} \cdot 100}$

Where ΔE_(control) is the ΔE of the thermoplastic starch compositioncontrol produced under a given composition and process but without anyreducing agent and ΔE_(agent) is the ΔE of the thermoplastic starchcomposition produced using the same composition and process but withreducing agent present. The value of ζ can vary from 100% for aperfectly white material to 0% for a material having the same color asthe control to less than 0% for a material of greater discoloration thanthe control.

Method D: Method for Producing Destructured Starch Using a Dry Process

A mixture of 1.5 kg of starch (ethoxylated corn starch from GrainProcessing Corporation (GPC) K-92F), 1.0 kg of sorbitol plasticizer (ADMTech Grade), 25 g of magnesium stearate (Spectrum Chemicals NC/FCC highpurity grade), and optionally 25 g reducing agent were mixed in a highspeed Henchel mixer operating at 10,000 RPM for five minutes. Themixture was then added to the feed throat of a 25 mm BP twin-screwcompounder outfitted with three degassing areas and 10 temperaturecontrol zones. The first degassing area was exposed to ambientconditions, whilst the other degassing areas were exposed to −28 cm Hg.The feed zone was set to 85° C. while the remaining zones were set toapproximately 150° C. The screw was set to 300 RPM. The strands exitingthe die were quenched in air and pelletized.

Method E: Method for Producing Destructured Starch Using a SlurryProcess

3.1 kg of de-ionized water was mixed with 2.24 kg of glycerol andstirred using a pneumatic paddle mixer operating at 150 RPM. Optionally,9.5 g of citric acid and 0.5 g of tribasic potassium citrate buffer wasadded to the water/glycerol mix and stirred for 1 minute. Optionally, 30g of reducing agent of the present invention was added to thewater/glycerol slurry while stirring. After 1 minute, 4 kg of starch(either modified starch from Tate and Lyle, Ethylex 2005S or naturalcorn starch from Tate and Lyle, Pure Food Grade Powder) was added to thewater/glycerol solution. Following 10 minutes of mixing at roomtemperature, the aqueous mixture was gravity fed to the hopper of a ZSK30 mm twinscrew extruder. The feed rate was adjusted to 1.8 kg/h. Thetwinscrew contained 12 temperature controlled barrel zones and onetemperature controlled die zone. The temperatures were set according tothe following temperature profile (° C.);

1 2 3 4 5 6 7 8 9 10 11 12 DH Ambient 95 100 104 138 161 161 161 161 161161 161 161

Those skilled in the art will understand how to set the temperatureprofile and operate the extruder. The screw speed was set to 100 RPM.The twinscrew contained distributive and dispersive mixing elementsbetween zones 1 and 3. An ambient pressure vent was contained in zone 4followed by additional distributive and dispersive mixing in zones 7through 10. Another ambient pressure vent was contained in zone 11. Zone12 contained metering/conveying elements followed by a three holesstrand die. Material exiting the die was air cooled and pelletized.

Method F: Method for Producing Thermoplastic Starch Compositions Blendedwith Other Thermoplastic Materials

2000 g of thermoplastic starch produced by Method D were dry blendedwith 1800 g of Dow 640i LDPE and 200 g of Dow Primacor 3460. The mixturewas added to the feed throat of a BP twin-screw compounder operating at300 RPM. The feed zone was set to 80° C. and the remaining zones wereset to 170° C. The strands exiting the die were quenched in a water bathand pelletized.

Method G: Method for Measuring the Molecular Weight of the StarchPresent in the Thermoplastic Starch Composition

Prior to testing samples of the present invention, a polysaccharide ofknown molecular weight was tested. Specifically, a low molecular weightnarrow dispersed polysaccharide (47,300 MW) from Polymer Laboratorieswas used. 0.024 g of the polysaccharide standard and 6.55 g of the DMSOmobile phase were added to a scintillation vial. This was leftovernight, after which the mixture was gently swirled then filter with a5 μm nylon syringe filter and injected into an autosampler vial.Standard concentration was set to be about 4 mg/ml.

If raw starch material (not de-structured) was used, 0.06 g of thesample was added to a 2 oz wide mouth jar. 22 g of the mobile phase wereadded and stirred using a stir bar for 5 minutes. The mixture was heatedfor 1 hour in an 85° C. oven. The samples were removed and allowed tosit overnight at room temperature. The mixture was filtered through a 5μm nylon syringe filter into an autosampler vial.

If thermoplastic starch was used, 3 mg/ml of the thermoplastic starch inmobile phases (those skilled in the art would know how to prepare amobile phase), was added to a 2 oz wide mouth jar. (Example: a 40%thermoplastic starch sample would have 0.15 g thermoplastic starch and22 g mobile phase). The above procedure was then followed.

Molecular weight was determined using the following equipment andmethod. A Waters 600E HPLC pump/system controller, a 717 autosampler, acolumn heater & controller, an in-line degasser, an Optilab DSPinterferometric refractometer, and a DAWN IOS 18 angle laser lightdetector were used. The column was a PL Gel 20 μm Mixed A type with alength of 600 mm and an inside diameter of 7.5 mm. The guard column wasa PL Gel 20 μm that was 50×75 mm. Those skilled in the art would knowhow to operate the equipment. The following settings and parameters wereused; Column Temperature: 55° C., Internal Temperature: 50° C., FlowRate: 1 ml/min, Run Time: 30 min, Laser Wavelength: 690 nm, Cell type:K5, Injection Volume: 200 μl, Mobile Phase: DMSO with 0.1% LiBr added,do/dc: 0.066. Mw is calculated from the following equation:

$M_{w} = \frac{\sum\limits_{i = 1}^{\#}{{n_{i} \cdot M}\; W_{i}^{2}}}{\sum\limits_{i = 1}^{\#}{{n_{i} \cdot M}\; W_{i}}}$

MW_(i) is the molecular weight of a particular polymer species, i. n_(i)is the number of that particular species having a MW_(i), and # is thetotal number of species in the polyethylene material.

During processing, the molecular weight of the starch is typicallyreduced due to chain scission and other complex reactions. Hydrolysisresults in a reduction in molecular weight and this is catalyzed by lowpH. An observed drop in Mw following processing is predominately causedby hydrolysis of the starch. The ability of the starch to maintain Mwduring processing is called the material process stability and isquantified by the stability index, τ, where τ is calculated from thefollowing equation:

$\tau = \left\lbrack \frac{\left( {{Mw}_{processed} - {Mw}_{virgin}} \right)}{{Mw}_{virgin}} \right\rbrack$

Where Mw_(virgin) is the weight average molecular weight of the initialstarch fed to the process, Mw_(processed) is the weight averagemolecular weight of the starch after processing. A τ of greater than 0%indicates an increase in molecular weight during processing and may beassociated with chain extension or crosslinking. A τ less than 0%indicates a decrease in molecular weight during processing and is likelyassociated with hydrolysis. The later is not preferred especially in thecase of products produced from extrusion blow molding, injection stretchblow molding, cast film, and blown film.

EXAMPLES

For all examples, the redox potential of the reducing agent was measuredusing Method A. The thermoplastic starch compositions were formed intopellets using Method D or E, and transformed into thin discs usingMethod B. Colour was measured using Method C.

Examples for Thermoplastic Starch Compositions Produced by the DryProcess and Using Low Molecular Weight Ethoxylated Starch

Thermoplastic starch compositions were prepared using Method D. Lowmolecular weight starch K-92F from Grain Processing Corporation wasused, and where applicable, reducing agent was added. A control wasproduced to establish the color of a thermoplastic starch producedwithout reducing agent. Results can be seen in Table 1 as Examples 1-4.Examples of thermoplastic starch compositions comprising reducing agentsoutside of the scope of the present invention are shown in Table 2, asExamples Comp A-C.

Examples for Thermoplastic Starch Compositions Produced by the SlurryProcess and Using Low Molecular Weight Ethoxylated Starch

Thermoplastic starch compositions were prepared using Method E. Lowmolecular weight starch Ethylex 2005S from Tate & Lyle was used, andwhere applicable, the reducing agent was added. A control was producedto establish the color of a thermoplastic starch composition producedwithout reducing agent. Results are shown in Table 1 as Example 5.

Examples for Thermoplastic Starch Compositions Produced by the SlurryProcess and Using Natural Corn Starch

Thermoplastic starch compositions were prepared using Method E. Naturalcorn starch, pure food grade powder from Tate & Lyle was used, and whereapplicable, reducing agent was added. Controls were produced toestablish the color of a thermoplastic starch compositions producedwithout reducing agent. The redox potential of the reducing agent wasmeasured using Method A. The thermoplastic starch compositions wereformed into pellets using Method D, and transformed into thin discsusing Method B. Colour was measured using Method C. Results are shown inTable 1 as Examples 6 and 7. An Example of a thermoplastic starchcomposition comprising reducing agent outside of the scope of thepresent invention is shown in Table 2, as Example Comp D.

TABLE 1 Color Redox Improve- Exam- Process Starch Poten- ment ple MethodType Reducing Agent tial Index ζ 1 D modi- disodium 2-hydroxy-2- −28052% fied sulfinatoacetic acid 2 D modi- thiourea dioxide −195 69% fied 3D modi- 2-hydroxy-2- −285 63% fied sulfonatoacetic acid 4 D modi- sodium−450 72% fied hydroxymethyl- sulfinate dihydrate 5 E modi- sodium −45053% fied hydroxymethyl sulfinate dihydrate 6 E natural sodium −450 39%hydroxymethyl sulfinate dihydrate 7 E natural disodium 2-hydroxy-2- −25034% sulfinato/sulfonato acetic acid 3:1 ratio mixture

TABLE 2 Color Comparative Process Starch Reducing Redox ImprovementExample Method Type Agent Potential Index ζ Comp A D modified taurine+145 −95%  Comp B D modified cysteic +200 −5% acid Comp C D modifiedsodium 0 16% sulfite Comp D E natural sodium +75  4% metabi sulfite

As can be seen from Tables 1 and 2, thermoplastic starch compositionscomprising reducing agents according to the present invention exhibitedimproved reduction of discoloration as indicated by the colourimprovement index, than in the absence of reducing agent or as comparedto reducing agents outside of the scope of the present invention.

Examples of Thermoplastic Starches According to the Present InventionBlended with Other Thermoplastics

Blends of thermoplastic starch compositions produced by Method D andanother thermoplastic were produced using Method F. Example 8 shown inTable 3 was produced using a thermoplastic starch composition containingsodium hydroxymethylsulfonate dihydrate via Method D.

TABLE 3 Color Redox Improve- Exam- Process Starch Poten- ment ple MethodType Reducing Agent tial Index ζ 8 D modified sodium −450 37%hydroxymethyl- sulfinate dihydrate

Effect of the Reducing Agent on the MW of the Starch Present in theThermoplastic Starch Composition

Thermoplastic starch compositions were prepared using the slurry process(Method E) at low pH with and without reducing agent to determine theimpact on the molecular weight of the starch. Natural corn starch, PureFood Grade Powder from Tate & Lyle was used. Results can be seen asExample 9 in Table 4. Comparative Example Comp E is also shown, whichcontains no reducing agent. The stability index τ was measured accordingto Method G.

TABLE 4 Example Reducing Agent pH Mw τ 9 sodium 2.8 19.9 +42%hydroxymethylsulfinate dihydrate Comp E none 2.5 2.0 −77%

As can be seen from Table 4, compositions according to the presentinvention exhibited higher molecular weight starch, as indicated by thestability index τ, than a composition not comprising a reducing agentaccording to the present invention.

The dimensions and values disclosed herein are not to be understood asbeing strictly limited to the exact numerical values recited. Instead,unless otherwise specified, each such dimension is intended to mean boththe recited value and a functionally equivalent range surrounding thatvalue. For example, a dimension disclosed as “40 mm” is intended to mean“about 40 mm.”

Every document cited herein, including any cross referenced or relatedpatent or application, is hereby incorporated herein by reference in itsentirety unless expressly excluded or otherwise limited. The citation ofany document is not an admission that it is prior art with respect toany invention disclosed or claimed herein or that it alone, or in anycombination with any other reference or references, teaches, suggests ordiscloses any such invention. Further, to the extent that any meaning ordefinition of a term in this document conflicts with any meaning ordefinition of the same term in a document incorporated by reference, themeaning or definition assigned to that term in this document shallgovern.

While particular embodiments of the present invention have beenillustrated and described, it would be obvious to those skilled in theart that various other changes and modifications can be made withoutdeparting from the spirit and scope of the invention. It is thereforeintended to cover in the appended claims all such changes andmodifications that are within the scope of this invention.

What is claimed is:
 1. A method for reducing discoloration ofthermoplastic starch compositions, said method comprising: including areducing agent as an ingredient in said thermoplastic starchcompositions, wherein the reducing agent has a redox potential fromabout −50 mV to about −1200 mV, the redox potential being measured in anaqueous solution at a temperature of about 80° C., said aqueous solutioncomprising about 1% reducing agent, about 1% citric acid, and about 1%tribasic potassium citrate by weight of the aqueous solution.
 2. Themethod of claim 1, wherein the reducing agent is a sulfur-comprisingreducing agent.
 3. The method of claim 2, wherein the sulfur-comprisingreducing agent is selected from the group consisting of sulfinic acidderivatives or salts thereof, sulfonic acid derivatives or saltsthereof, and mixtures thereof.
 4. The method of claim 3, wherein thesulfur-comprising reducing agent is selected from the group consistingof sodium hydroxymethylsulfonate, disodium 2-hydroxy-2-sulfinatoaceticacid, sodium dithionite, thiourea dioxide, disodium2-hydroxy-2-sulfonatoacetic acid, and mixtures thereof.
 5. The method ofclaim 4, wherein the sulfur-comprising reducing agent is selected fromthe group consisting of disodium 2-hydroxy-2-sulfinatoacetic acid,disodium 2-hydroxy-2-sulfonatoacetic acid, and mixtures thereof.
 6. Themethod of claim 1, wherein the redox potential of the reducing agent hasa value of from about −100 mV to about −1200 mV.
 7. The method of claim6, wherein the redox potential of the reducing agent has a value of fromabout −150 mV to about −1200 mV.
 8. The method of claim 1, wherein saidthermoplastic starch composition comprises: (a) from about 40% to about96% starch, by weight of the thermoplastic starch composition; (b) fromabout 1% to 40% plasticizer, by weight of the thermoplastic starchcomposition; and (c) from about 0.01% to about 5% reducing agent, byweight of the thermoplastic starch composition.
 9. The method of claim8, wherein the starch is selected from the group consisting of naturalstarch, modified starch, and mixtures thereof.
 10. The method of claim9, wherein the starch is a natural starch.
 11. The method of claim 9,wherein the starch is a modified starch.
 12. The method of claim 8,wherein the thermoplastic starch composition comprises anotherthermoplastic material.
 13. The method of claim 12, wherein the otherthermoplastic material is selected from the group consisting ofpolyethylene, polypropylene, polyethylene terephthalate and mixturesthereof.
 14. The method of claim 13, wherein the thermoplastic starchcomposition additionally comprises added water.
 15. The method of claim13, wherein the pH of the thermoplastic starch composition is adjustedto less than about
 6. 16. The method of claim 15, wherein the pH of thethermoplastic starch composition is adjusted to less than about 4.